The present invention relates to drawn and oriented surgical filaments and allied surgical products, and more particularly to strong, but flexible monofilament sutures having unique handling and knot tying characteristics. The novel sutures and surgical products of the present invention comprise an oriented copolymer consisting of recurring poly(polymethylene terephthalate, isophthalate or cyclohexane-1,4-dicarboxylate) and poly(polymethylene dimerate) units.
Many natural and synthetic materials are presently used as surgical sutures. These materials may be used as single filament strands, i.e., monofilament sutures, or as multifilament strands in a braided, twisted or other multifilament construction. Natural materials such as silk, cotton, linen, and the like, do not lend themselves to the fabrication of monofilament sutures and are accordingly generally used in one of the multifilament constructions.
Certain synthetic materials which are extruded in continuous lengths can be used in monofilament form. Common synthetic monofilament sutures include polypropylene, polyethylene and nylon. Such monofilament sutures are preferred by surgeons for many surgical applications due to their inherent smoothness and noncapillarity to body fluids.
Available synthetic monofilament sutures all suffer to a greater or lesser degree from one particular disadvantage, that is relative stiffness. Besides making the material more difficult to handle and use, suture stiffness or low compliance can adversely affect knot tying ability and knot security. It is because of the inherent stiffness of available monofilament sutures that many suture materials are braided or have other multifilament constructions with better handling, flexibility and conformity.
Most monofilament sutures of the prior art are also characterized by a low degree of compliance. This makes knot tying difficult and reduces knot security. In addition, the low compliance and limited ductility prevent the suture from "giving" as a newly sutured wound swells, with the result that the suture may place the wound tissue under greater tension than is desirable, and may even cause some tearing, cutting or necrosis of the tissue.
The problems associated with the use of low compliance sutures in certain applications were recognized in U.S. Pat. No. 3,454,011, where it was proposed to fabricate a surgical suture composed of Spandex polyurethane. Such sutures, however, were too elastic and did not find general acceptance in the medical profession.
Recently issued U.S. Pat. No. 4,224,946 describes a monofilament suture with good flexibility and knot strength, which suture is composed of block polyetheresters which contain (1) a polymeric block of polyalkylene ethers and (2) a polymeric block of aromatic dicarboxylic acids or cycloaliphatic acids with short chain aliphatic or cycloaliphatic diols. Similar subject matter is disclosed in Belgian Pat. No. 880,486.
Copolyesters of aromatic diacids (e.g. terephthalic acid) and "dimer acids" of C.sub.18 unsaturated fatty acids have been known for some time in the technical and patent literature.
Hoeschele [Angew.Makromol.Chem. 58/59, 229(1977)] as well as in U.S. Pat. No. 3,954,689 disclosed the preparation of thermoplastic PBT (polybutylene terephthalate)/dimerate systems for the generation of elastomeric films and molded articles which were largely unoriented. However, no reference was made to the conversion of these copolymers to oriented fibers, which would possess mechanical properties suitable for use as flexible monofilament sutures.
In connection with the Hoeschele U.S. Pat. No. 3,954,689, discussed above, all of the features stressed by Hoeschele relate to those that one would associate with rubbers ( and not drawn fibers) i.e., flexibility, abrasion, resistance, etc. and with coating materials and with materials suitable for molded articles. The fact that Hoeschele's materials are suitable for molding does not imply that they are necessarily suitable for spinning (for example, butyl rubber cannot be used to make fibers). Although Hoeschele covers a broad range with respect to his "hard" and "soft" components, which ratio encompasses the presently claimed ratio thereof, nevertheless there is no specific disclosure of the presently claimed ratio range. Accordingly, even if an attempt were to be made to produce fibers from the Hoeschele copolymers, there is no specific teaching by Hoeschele of the optimum ratio range found by the present applicant to be essential for the production of suitable surgical filaments and accordingly the Hoeschele disclosure is deficient in this regard.
In accordance with the present invention there is provided a drawn and oriented, flexible thermoplastic surgical filament which has an unusual combination of acceptable knot strength and high compliance, an integration of properties that is non-existent in commercial monofilament sutures. Accordingly, the two critical parameters required in accordance with the present invention are the Young's modulus (which is the reciprocal of compliance) and knot strength, the latter being the most important parameter. The range of compositions disclosed by Hoeschele in U.S. Pat. No. 3,954,689 for the "rubbery" material disclosed therein is broad, but absolutely no indication is provided by Hoeschele concerning any critical range of compositions which could provide a balance of good knot strength and high compliance, which is not surprising since his invention was totally unrelated to drawn fibers. For a useful surgical suture of common diameter such as size 3/0, it is critical that the knot strength be at least 35,000 psi. On the other hand, in a uniquely compliant useful surgical suture, the Young's modulus should be between about 80,000 and 280,000 psi. In this connection it was found, in accordance with the present invention, that a copolymer composition containing between 8 and 13 mole % of the dimerate component is absolutely essential in order to provide the unique combination of properties discussed above. The criticality of the particular compositions dependent upon the two parameters of Young's modulus and knot strength was further verified by preparing and testing a homopolymer (i.e., a polymer containing 0% of the dimerate), as well as a copolymer containing 84.275 mole % of the "hard" component and 15.725% of the "soft" dimerate component (which is the same as the copolymer prepared in accordance with Example I of the Hoeschele U.S. Pat. No. 3,954,689) in order to compare the properties thereof with the presently claimed range of 8-13 mole % of the dimerate component. The properties of the aforementioned hopolymer and copolymer known from the Hoeschele patent are set forth in present Example VI. Example I of the Hoeschele patent was selected for comparison purposes since the product thereof represents the closest composition, exemplified in Hoeschele, to that presently claimed, as a fiber-former.
As can be seen from the table in Example VI fibers prepared from both the Hoeschele copolymer X and the homopolymer Y possess acceptable and almost identical straight tensile strengths. Although the homopolymer Y has good knot strength, its compliance is unacceptable, for it is about 3 times as stiff as applicant's preferred suture which contains 10% of the "soft" component as indicated by samples 9-12 in Table II wherein the Young's modulus varies between 126,000 psi and 162,000 psi. Although the Hoeschele copolymer X has a low Young's modulus, it possesses a completely unacceptable knot strength (for a size 3/0 strand) of only 28,427 psi. Samples 9-12 of the present invention, as indicated in Table II herein, possess, on the other hand, knot strengths ranging between 39,000 psi and 41,000 psi. This difference between the knot strength of copolymer X and that of the presently claimed copolymer, is surprising, since the mole percentages of the "soft" dimerate components of the two chains in question are only slightly different. The selection of the minimum knot strength is in accordance with the USP requirements for nominal size 3/0 sutures. Specifically, the value of 35,750 psi is the intrinsic knot strength corresponding to the mean of the two values of intrinsic strength calculated using the "Average Knot-Pull Tensile Strength" and Minimum and Maximum Average Diameters for size 3/0 as found on page 714 of the fourth supplement of the USPXX Official Monograph [1983].
Data with respect to the homopolymer Y in Example VI herein is given in order to illustrate the fact that when the mole percentage of the "soft" component reaches 0 the resultant homopolymer is unacceptably stiff. It has been found, in accordance with the present invention that the mole percentage range of the "soft" component which provides the required properties of the present invention, is surprisingly narrow.
According to a number of patents [U.S. Pat. No. 3,390,108 (1968), U.S. Pat. No. 3,091,600 (1963) and British Pat. No. 994,441 (1965)], PET (polyethylene terephthalate) copolymers containing small amounts of dimerate moieties were spun into fibers which possessed superior dying properties. In accordance with U.S. Pat. No. 3,649,571, polyethylene terephthalate is rendered dyeable by incorporating therein the product of the reaction of a small amount of dimer acid with the sodium salt of isothionic acid. Since the processes of dye takeup and diffusion during conventional external dyeing, invariably take place in amorphous regions of a polymer, the observed enhancement of dyeability of the modified PET fibers may be attributed to a lowering of crystallinity by the dimerate structure. At the dimerate levels used in these compositions, the mechanical properties of the fibers are far from compatible with those applications which are sought for sutures, for at least their low compliance (or high Young's Modulus).
In the few instances in which higher concentrations of dimerate were incorporated into PET copolymers [Belgium Pat. No. 649,158 (1964), U.S. Pat. No. 3,383,343 (1968), and French Pat. No. 1,398,551 (1965)], the resulting fibers exhibited inferior tensile properties which rendered these products unsuitable as potential suture materials.
In view of the above discussion, the prior art holds little promise for the utilization of a polyester/dimerate polymer as basis for a suture material. Furthermore, since dimer acids are long chain branched molecules, the findings in the literature were not surprising, as theory and experience in the art of fiber science predict that branching exerts a deleterious effect on the tensile properties of the resulting fibers. The reason for this is (a) the inability of the branch to confer good tensile properties, which by its very nature can neither be easily oriented along the fiber axis nor contribute to the load bearing capacity of the fiber required to counteract mechanical stresses and (b) the steric interference posed by the branch to the main chain alignment during fiber orientation.
It is an object of the present invention to provide a novel flexible, thermoplastic monofilament suture or ligature of poly[polymethylene terephthalate, isophthalate or cyclohexane-1,4-dicarboxylate-co-dimerate], having a diameter of from about 0.01 to 1.0 mm and possessing unique and desirable physical properties. A further object of the present invention is to provide a filament which suffers practically no losses in physical properties as judged by a comparison of inherent viscosities and tensile strength before and after Co.sup.60 sterilization (2.5 megarads). It is yet a further object of the present invention to provide a filament which possesses superior thermal oxidative stability as compared with more conventional types of low modulus thermoplastics such as those denoted as polyetheresters (this is because of the inherent instability of the latter). Another object is to provide a filament having improved dye retention, when utilizing dispersed-type dyes in a typical melt dying process. These and other objects will be made apparent from the ensuing description and claims.